Vibration isolator



y 7, 1949. E. PIETZ v VIBRATION ISOLATOR 2 Sh e etS-Sheet 1 Filed May 29, 1945 Inventor Bruin Pietz Fig. 3

2 Sheets-Sheet 2 Fz 'g. 10

Inventor-- vz'n Pietz May 17, 1949. E. PIETZ VIBRATION ISOLATOR Filed May 29, 1945 Er MMA 2.

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Patented May 17,1949

VIBRATION ISOLATOR Ervln Pietz, Cambridge, Mass., assignor to Lawrence N. Barry, George W. Foss, and Ervin Pietz, doing business as The L. N. Barry Company,

Cambridge, Mass.

Application May 29, 1945, Serial No. 596,483

2 Claims.

My invention relates to vibration isolators; i. e., means to greatly reduce the magnitude of vibrations transmitted to delicate apparatus or to prevent the vibrations induced by rotating or reciprocating machinery from causing disturbances in adjacent equipment.

An isolator of the type disclosed herein has particular application in aircraft for the purpose of protecting electronic and other delicate apparatus from the destructive vibrations caused by the action of the engines and propellers, and by the air of the slipstream impinging upon the structure of the airplane. My invention has fur her application to aircraft for mounting the engines to prevent the transmission of vibration to other parts of the airplane. A'similar application is in the mounting of automobile engines. Additional applications are in the mounting of delicate equipment in automobiles, trucks and railway trains and in isolating the motor vibrations in washing machines, refrigerators, and similar equipment.

The mounting of electronic equipment in aircraft involves a special problem. Maintenance,

practice on equipment of this type dictates that the equipment be readily removable and replaceable. As a result, the equipment is generally set upon a tray which is attached to the aircraft by means of vibration isolators located underneath the tray. Airplane engines operate within a wide range of speeds, above a predetermined minimum. To properly prevent transmission of the vibration from such engines to the delicate equipment, it is essential that the natural frequency of the equipment upon the isolators in all natural modes of vibration be less than the minimum frequency of engine vibration. It is an object of my invention, then, to provide a mount which has a low natural frequency in all natural modes of vibration when located underneath the equipment to be isolated.

As will hereinafter be described in detail, an

isolator located underneath the equipment has natural frequencies low in all natural modes of vibration only if the lateral stiffness of the mount is low compared with the vertical stiffness. It is another object of my invention, then, to arrange the rubber parts in such a manner that lateral stiffness can be made very low; 1. e., the lateral flexibility is high.

Another object of my invention is to design a mount which has a minimum height together with a stiffness low enough for supporting very light equipment with the required natural frequencies. The stiffness of a piece of rubber can generally be reduced by increasing the thickness. I accomplish such an effective increase in thickness without increasing the dimensions by using ovleirlapping or staggered resilient elements in se cs.

A further object of my invention is to maintain a high stability of the mount by employing a plurality of resilient elements in series and in staggered relation to one another. Such an arrangement has greater stability than a single resilient element of equivalent stiffness.

A still further object of my invention is to design a vibration isolator which utilizes solid rubber in place of sponge rubber. Sponge rubber has a high drift rate, particularly at elevated temperatures; it is susceptible to damage if moisture freezes in the pores; it deteriorates rapidly when exposed to ozone; and it lacks consistency in manufacture. However, it has a much lower stiffness than solid rubber which makes it useful for supporting light weight equipment. I attain an equally low stiffness using solid rubber by forming the rubber parts with a small load-carrying area and a large surface over which the rubber is free to bulge or expand. This form factor, together with the staggered or overlapped elements, enables me to successfully mount equipment in the weight range for which sponge rubber elements are ormally used.

Rubber and similar materials normally used f or the resilient elements in vibration isolators of the type disclosed slowly drift or compress when subjected to a constant load. This drift increases with an increase in unit strain or unit static defiection. Since there is a fixed relation between natural frequency and total strain (static deflection) in a resilient system, the total strain or static deflection is determined by the operating requirements for the isolator. It is an object of my invention to reduce the drift by reducing the unit strain. This is accomplished, while holding' the total strain constant, by increasing the total thickness of rubber.

It is necessary that vibration isolators used in aircraft and certain other types of machines have means to limit total motion of the mounted equipment. When landing, airplanes experience shock motions which would cause equipment within theairplane to impact against adjacent structures unless restraining means are employed. I provide such restraining means, hereinafter called snubbers or secondary resilient elements, and locate such snubbers adjacent the perimeter of the equipment so that they function as efliciently as possible'in preventing tipping. The supporting elements, or primary resilient elements, are located inwardly of the perimeter to give better natural frequency conditions.

snubbers are effective in preventing large amplitudes of vibration at the resonant frequency of the flexible element, such as when the airplane engine is being accelerated from idling to operating speed or when the motor of a refrigerator passes through the resonant frequency of the mounts in coming up to speed. When the clearance to the snubber is small and the snubber is made of an undamped material, bouncing between the top and bottom snubbers is experienced at resonance. This is undesirable because large ,stresses may be caused in the equipment by the violent snubber action. It is an object of my invention to provide damped snubbers so that the rebound from each snubber is reduced in velocity and the severity of the snubber action is consequently reduced.

When the frequency of the forcing vibration is increased so as to pass through the resonant frequency of the vibration isolators, the amplification increases due to resonant action and bouncing on the snubbers occurs. In the case of the conventional, undamped snubbers, bouncing continues as a pseudo-resonant vibration even though the frequency has increased well beyond the actual resonant frequency. As a further object of my invention, I provide damped snubbers to prevent pseudo-resonant vibrations throughout a wide frequency range.

Another object of my invention is to provide unit mounts which can be conveniently attached to equipment of odd shapes, such as cylindrical containers. I

Other objects and advantages of my invention will become apparent from the following detailed description by referring to the accompanying drawings in which:

Figure 1 is a perspective view showing'the application of my invention to the isolation of vibration from aircraft equipment.

Figure 2 is a partial plan view on line 2-2 of Figure 1.

Figure 3 is a section on line 3-3 of Figure 2.

Figure 4 is a section on line 4-4 of Figure 2.

Figure 5 is a. view similar to Figure 4 showing the vibration isolator deflected laterally.

Figure 6 is a graph showing the natural frequencies in several modes of vibration of equipment mounted as shown in Figure 1.

Figure '7 shows a modified form of my invention applied to acylindrically shaped equipment.

Figure 8 is an enlarged sectional view of a portion of Figure 7.

Figures 9, 10, and 11 are views similar to Figure 8 showing several modified forms of my invention.

As shown in Figure 1, the'equipment ID to be isolated is set upon the vibration isolator and secured thereto in any convenient manner.

The isolator is comprised of two downwardly facing upper channels H and two upwardly facing lower channels [3, the upper channels I I preferably being joined'by crossvmembers M to form an integral unit. The equipment in is secured to the cross members I4 or upper channels H. The lower channels 13 are rigidly attached to the structure of the airplane or other foundation.

In the embodiment of my invention illustrated, four flexible load-carrying assemblies H are provided. Each of the assemblies ll includes a lower thin-walled cylindrical element I9, preferably rubber or rubber-like material, positioned by the rigid ring 20 secured to a lower channel l3. The rigid intermediate piece 22 has an outer annular part 23 resting upon the upper face of th resilient'element l9 and a central part 25 which protrudes downwardly into the hollow center of the resilient element l9. An upper cylindrical element 26, also of rubber or rubber-like material, rests upon the central part 25 of the intermediate piece 22 and supports, upon its upper face, the upper channel II. The resilient elements may 4 I be metallic'helical springs in place of rubber cylinders. The upper channel II is positioned upon the upper resilient element 28 by a pin 28 rigidly attached to the channel II and extending downwardly into the central recess of the element 26. The outer annular part 23 and the central part 25 of the intermediate piece 22 are joined by a connecting part 3| which is flared downwardly and inwardly to permit shearing and tipping of the resilient elements I9, 25 as shown in Figure 5 when the upper channel moves laterally relatively to the lower channel I3. The flexible load-carrying assemblies 11 are preferably spaced inwardly from the ends of the channels I9, 26 for reasons to be hereinafter described.

The upper (H) and lower (l3) channels are held together by the snubbing assemblies 33 located adjacent the ends of the channels H, B. Each such assembly comprises a bridge 34 attached to the lower channel l3 and a pin 36 secured to the upper channel ll and depending through a clearance hole 31 in the bridge 34. A washer 38, larger than the hole 3! in the bridge 34, is fixed to the lower end of the pin 36. A grommet 4|! is slidably fitted to the pin 36 and the central groove 42 of the grommet receives the bridge 34. The upper (43) and lower (39) washers engage the grommet after predetermined relative motion between upper (II) and lower (l3) channels. The grommet 40 is preferably resilient to provide a cushioned limiting device and preferably made of a higly damped material such as felt (rubber impregnated for protection), synthetic rubber, or other suitable material. The

damping functions to deaden the rebound from the snubbers and thereby reduces the severity of vibrations which involve snubber action. The upper (26) and lower (l9) resilient members are substantially undamped; i. e., the damping is be very flexible. Vibration isolators for aircraft have heretofore-been made of sponge rubber because of the softness of such material. However,

there are disadvantages, described above, associated with Sponge rubber. By forming the rubber parts in a novel manner, I use solid rubber and obtain a sufliciently low natural frequency. It is shown in the paper Rubber springs, by Walter C. Keys, in the May, 1937, issue of Mechanical Engineering (pages 345 to 349) that the stiffness of a rubber part decreases as the ratio of load-carrying area, to bulge area decreases. By forming the rubber parts as hollow cylinders, I maintain this ratio low and consequently employ rubber parts which are soft. Furthermore, such a cylinder is very stable under a vertical load and has ability to deflect laterally in shear as shown in Figure 5. This method of deflection makes possible a low lateral stiffness.

Figure 6 is a graph-showing the relative values of three natural frequencies of a unit of equipment mounted upon a vibration isolator. The isolator is located underneath the equipment, as shown in Figure 1, and the resilient assemblies are spaced inwardly from the ends of the channel members I I, I3. The horizontal line 46 in Figure 6 marked vertical mode is the natural frequency in a vertical translational mode which is independent of the lateral stiffness of the isolator. The other two lines 41, 48 represent the natural frequencies in rocking modes about two member is an inverted cup is having a base flange mutually parallel axes. It is one of the necessary conditions to vibration isolation that the natural frequencies in all modes be substantially below the frequency of the vibration to be isolated. The natural frequency in the higher rocking mode (line ll in Figure 6) is the troublesome one in the kept sumciently low only by maintaining a low lateral mount stiffness relative stiffness. It is this consideration which dictates many features of the mounts-the thin hollow cylindrical rubber elements and the oflset central metal piece formed with flared flanges and intermediate parts to permit ready lateral deflection of the rubber elements. This enables me to maintain the ratio of lateral to vertical stiffness low and the spread of natural frequencies consequently low. The maximum emciency oi vibration isolation is thus attained.

Figures 7 and 8 show an embodiment of my invention which takes the form of a unit mount iiii. Such a mount has general purpose applications but is particularly applicable to equipment or odd shape, such as the cylin iii. The mount it comprises a hollow cylindrical resilient element preferably made oi solid. rubber; a rigid intermediate piece as having its central part [56 extending downwardly into the hollow element 53; and the upper hollow resilient element 51 resting upon the central part es of the piece it. The mount 50 is attached to the equipment ea by means of the stud so attached at its lower end to the piece 5d and at its upper end to from the by means of abolt through the central tube 6 which is attached at its upper end to the OOVi i plate 94. The snubber is in the form of tww washers, one compressed between the cover plate 913 and the cup 90-when a downward force is applied to the mount. The lower snubbing washer. 99 is compressed, when an upward force is applied, be tween the cup 90 and the member i0l attached to the vertical 10 to the lower end-of the tube 96. The lower washer 99 ls-also efiective when a horizontal force is applied. The snubbers are preferably made of a resilient, highly damped material.

Figure 11 illustrates a still further modification of my invention in which the load is carried from the central stud I02 through the bonded 4 rubber disc I03 to the inverted cup I04. A secondary cup i05 fits underneath the cup 104 to provide rigid i9! attached (91) overlying the cup 90 and being restraint for the snubbing grommet to the central stud I02. The grommet new is preferably made of resilient, highly damped material.

The accompanyin drawings illustrate the proierred form of the invention, though it is to be 523%] equipment 25 understood that the invention is not limited to the exact details of construction shown and de scribed, as it is obvious that various modifies tions thereof, within the scope of the claims. will occur to persons skilled in the art.

I claim:

1. Means adapted to be interposed between two bodies for the purpose of isolating vibration one other comprising an upper cylindrical rubber element in at least partial telescoped role-- the bracket 6t extending outwardly from the tion witha lower cylindrical rubber element, and

equipment at. The mount is enclosed by the inverted cup 62 having its lower flange as attached to the base plate 55 of the mount and its upper flange so overlying the outer flange 68 of the inan interposed relatively rigid piece engaging both of said elements so that the weight of said body is carried by said elements in series, said piece having inclined walls out of contact with said \term'ediate piece at, thereby retaining the piece elements whereby said elements are free to de 5% and consequently the equipment 5|. The upper flange is fitted with a snubber 69 which ongages the flange to on its lower side and the washer ii on its upper side to limit motion across the mount. of (a resilient material having high damping capacity such as felt (impregnated with rubber for protection). a synthetic rubber having high damping capacity, or other suitable material. bodying my invention. The main supporting it for securing it to the foundation and an up wardly projecting central part it. The upper resilient element it rests upon the outer shoulder of the cup i5 and supports the downward load imposed by the washer ti. A lower resilient ele ment 82 fits within the central part it of the cup is and resists any upward force imposed by the central stud es and the lower washer to. Snub- 30 bers are in the form of annular rings bl, as arranged to be engaged after a predetermined up- Number ward or downward motion across the mount. 919 37 The snubber rings all, 88 are preferably made 1,030,74

or highly damped material to minimize bouncing 2,195, 30

action. 2,359,917

In the modification shown in Figure 10, the 2,3 7,597 main supporting member is an inverted cup 90 2,377,492 to which is bonded the load-carrying resilient 2,397,304

element 9|. The resilient element 9! is in the ,4 10% form of an annular ring. An outer cylindrical member 93 is also bonded to the resilient element 9i and a cover plate 94 is crimped onto the cylindrical member 93. The equipment is attached Number lation thereto, and

he both of so d body is carried piece engagi Figure 9 shows an alternative construction can 50 weight f al flect laterally to maintain all natural frequencies of the system low.

2. Means adapted to be interposed between two bodies for the purpose of isolating vibration one The snubber 69 is prefera ly m d 45 from the other comprising an upper cylindrical rubber element nesting at least partially within a lower cylindrical rubber element in spaced re-: an interposed relatively rigid id elements so that the by said elements. in series, said piece having inclined walls out of contact with said elements whereby said elements are free to deflect laterally to maintain all natural frequencies of the system low.

YEN rm'rz.

BEEFEEENEES GET UNITED STATES i N mo Jaiivet Apr. 27, i Chatain Dec. 9, 1983 hiarzeth 26, 1940 Hussxnan Oct. 10, 1944 Stita et al. Jan. 23 uses Ger-ton ulune 5, 1945 Nairken et al Apr. 2, 1948 Thiry Mar. ill, 194:7 FOREIGN PA l Country to France u 

